Hybrid electric vehicle

ABSTRACT

A hybrid electric vehicle drive system comprising a combustion engine, an electric motor and at least one nickel-metal hydride battery module forming a power source for providing electric power to the electric motor, the at least one nickel-metal hydride battery module having a peak power density in relation to energy density as defined by: 
     
       
           P&gt; 1,375−15 E    
       
     
     where P is the peak power density as measured in Watts/kilogram and E is the energy density as measured in Watt-hours/kilogram.

RELATED APPLICATION INFORMATION

The present invention is a continuation of U.S. patent application Ser.No. 08/979,340 filed on Nov. 24, 1997 now U.S. Pat. No. 6,330,925, whichis a continuation-in-part of U.S. patent application Ser. No. 08/792,358now U.S. Pat. No. 5,856,047 and Ser. No. 08/792,359 now U.S. Pat. No.5,851,698, both filed Jan. 31, 1997. U.S. patent application Ser. No.08/979,340 is hereby incorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates generally to a hybrid electric vehicleincorporating an integrated propulsion system. More specifically, thisintegrated propulsion system comprises a combustion engine, an electricmotor, high specific power, high energy density nickel-metal hydride(NiMH) batteries, and preferably a regenerative braking system. The NiMHbatteries of the invention have negative electrodes with substrates ofenhance current collecting capabilities, positive electrodes havingenhance conductivity, and other improvements to enhance the powerdelivery capability of the battery and permit maximum operatingefficiency during charge and discharge cycling while maintaining acombination of energy density and power density which provides enhancedperformance beyond the capabilities of prior art NiMH battery systems.

BACKGROUND OF THE INVENTION

Advanced automotive battery development for vehicle propulsion has, inthe past, been directed primarily at the requirement of fully electricpropulsion systems for such vehicles. To this end, Stanford Ovshinskyand his battery development teams at Energy Conversion Devices, Inc. andOvonic Battery Company have made great advances in nickel-metal hydridebattery technology for such applications.

Initially effort focused on metal hydride alloys for forming thenegative electrodes of such batteries. As a result of their efforts,they were able to greatly increase the reversible hydrogen storagecharacteristics required for efficient and economical batteryapplications, and produce batteries capable of high density energystorage, efficient reversibility, high electrical efficiency, efficientbulk hydrogen storage without structural changes or poisoning, longcycle life, and repeated deep discharge. The improved characteristics ofthese highly disordered “Ovonic” alloys, as they are now called, resultsfrom tailoring the local chemical order and hence the local structuralorder by the incorporation of selected modifier elements into a hostmatrix.

Disordered metal hydride alloys have a substantially increased densityof catalytically active sites and storage sites compared to single ormulti-phase compositionally homogeneous crystalline materials. Theseadditional sites are responsible for improved efficiency ofelectrochemical charging/discharging and an increase in electricalenergy storage capacity. The nature and number of storage sites can evenbe designed independently of the catalytically active sites. Morespecifically, these alloys are tailored to allow bulk storage of thedissociated hydrogen atoms at bonding strengths within the range ofreversibility suitable for use in secondary battery applications.

Some extremely efficient electrochemical hydrogen storage materials wereformulated, based on the disordered materials described above. These arethe Ti—V—Zr—Ni type active materials such as disclosed in U.S. Pat. No.4,551,400 (“the '400 Patent”) to Sapru, Hong, Fetcenko, and Venkatesan,the disclosure of which is incorporated by reference. These materialsreversibly form hydrides in order to store hydrogen. All the materialsused in the '400 Patent utilize a generic Ti—V—Ni composition, where atleast Ti, V, and Ni are present and may be modified with Cr, Zr, and Al.The materials of the '400 Patent are multiphase materials, which maycontain, but are not limited to, one or more phases with C₁₄ and C₁₅type crystal structures.

Other Ti—V—Zr—Ni alloys are also used for rechargeable hydrogen storagenegative electrodes. One such family of materials are those described inU.S. Pat. No. 4,728,586 (“the '586 Patent”) to Venkatesan, Reichman, andFetcenko, the disclosure of which is incorporated by reference. The '586Patent describes a specific sub-class of these Ti—V—Ni—Zr alloyscomprising Ti, V, Zr, Ni, and a fifth component, Cr. The '586 Patent,mentions the possibility of additives and modifiers beyond the Ti, V,Zr, Ni, and Cr components of the alloys, and generally discussesspecific additives and modifiers, the amounts and interactions of thesemodifiers, and the particular benefits that could be expected from them.

In contrast to the Ovonic alloys described above, the older alloys weregenerally considered “ordered” materials that had different chemistry,microstructure, and electrochemical characteristics. The performance ofthe early ordered materials was poor, but in the early 1980's, as thedegree of modification increased (that is as the number and amount ofelemental modifiers increased), their performance began to improvesignificantly. This is due as much to the disorder contributed by themodifiers as it is to their electrical and chemical properties. Thisevolution of alloys from a specific class of “ordered” materials to thecurrent multi-component, multiphase “disordered” alloys is shown in thefollowing patents: (i) U.S. Pat. No. 3,874,928; (ii) U.S. Pat. No.4,214,043; (iii) U.S. Pat. No. 4,107,395; (iv) U.S. Pat. No. 4,107,405;(v) U.S. Pat. No. 4,112,199; (vi) U.S. Pat. No. 4,125,688 (vii) U.S.Pat. No. 4,214,043; (viii) U.S. Pat. No.4,216,274; (ix) U.S. Pat. No.4,487,817; (x) U.S. Pat. No. 4,605,603; (xii) U.S. Pat. No. 4,696,873;and (xiii) U.S. Pat. No. 4,699,856. (These references are discussedextensively in U.S. Pat. No. 5,096,667 and this discussion isspecifically incorporated by reference). Additional discussion iscontained in the following U.S. patents, the contents of which arespecifically incorporated by reference: U.S. Pat. Nos. 5,104,617;5,238,756; 5,277,999; 5,407,761; 5,536,591; 5,506,069; and 5,554,456.

Ovshinsky and his teams next turned their attention to the positiveelectrode of the batteries. The positive electrodes today are typicallypasted nickel electrodes, which consist of nickel hydroxide particles incontact with a conductive network or substrate, preferably having a highsurface area. There have been several variants of these electrodesincluding the so-called plastic-bonded nickel electrodes which utilizegraphite as a microconductor and also including the so-called foam-metalelectrodes which utilize high porosity nickel foam as a substrate loadedwith spherical nickel hydroxide particles and cobalt conductivityenhancing additives. Pasted electrodes of the foam-metal type havestarted to penetrate the consumer market due to their low cost andhigher energy density relative to sintered nickel electrodes.

Conventionally, the nickel battery electrode reaction has beenconsidered to be a one electron process involving oxidation of divalentnickel hydroxide to trivalent nickel oxyhydroxide on charge andsubsequent discharge of trivalent nickel oxyhydroxide to divalent nickelhydroxide. However, quadrivalent nickel is also involved in the nickelhydroxide redox reaction, the utilization of which has never beeninvestigated.

In practice, electrode capacity beyond the one-electron transfertheoretical capacity is not usually observed. One reason for this isincomplete utilization of the active material due to electronicisolation of oxidized material. Because reduced nickel hydroxidematerial has high electronic resistance, the reduction of nickelhydroxide adjacent the current collector forms a less conductive surfacethat interferes with the subsequent reduction of oxidized activematerial that is farther away.

Ovonic Battery Company has developed positive electrode materials thathave demonstrated reliable transfer of more than one electron per nickelatom. Such stable, disordered positive electrode materials are describedin U.S. Pat. Nos. 5,344,728; 5,348,822; 5,523,182; 5,569,563; and5,567,599; the contents of which are specifically incorporated byreference.

As a result of this research into the negative and positive electrodeactive materials, the Ovonic Nickel Metal Hydride (NiMH) battery hasreached an advanced stage of development for EVs. Ovonic electricvehicle batteries are capable of propelling an electric vehicle to over370 miles (due to a specific energy of about 90 Wh/Kg), long cycle life(over 1000 cycles at 80% DOD), abuse tolerance, and rapid rechargecapability (up to 60% in 15 minutes). Additionally, the Ovonic batteryhas demonstrated higher power density when evaluated for use as an EVstored energy source.

As an alternative to true electric vehicles, hybrid-electric vehicles(HEVs) have gained popularity as having the technical capability to meetthe goal of tripling auto fuel economy in the next decade. Hybridvehicles utilize the combination of a combustion engine and an electricmotor driven from a battery and have been proposed in a variety ofconfigurations.

Hybrid systems have been divided into two broad categories, namelyseries and parallel systems. In a typical series system, an electricpropulsion motor is used to drive the vehicle and the engine is used torecharge the battery. In a parallel system, both the combustion engineand the electric motor are used to drive the vehicle and can operate inparallel for this purpose.

There are further variations within these two broad categories. Forexample, there are systems which employ a combination of the series andparallel systems. In the so-called “dual mode” system, the propulsionmode can be selected, either by the operator or by a computer system, aseither an “all electric” or “all engine” mode of propulsion. In the“range extender” system, a primarily electric system is used forpropulsion and the engine is used for peak loads and/or for rechargingthe battery. In the “power assist” system, peak loads are handled by thebattery driven electric motor.

A further division is made between systems which are “charge depleting”in the one case and “charge sustaining” in another case. In the chargedepleting system, the battery charge is gradually depleted during use ofthe system and the battery thus has to be recharged periodically from anexternal power source, such as by means of connection to public utilitypower. In the charge sustaining system, the battery is recharged duringuse in the vehicle, through regenerative braking and also by means ofelectric power supplied from a generator driven by the engine so thatthe charge of the battery is maintained during operation.

There are many different types of systems that fall within thecategories of “charge depleting” and “charge sustaining” and there arethus a number of variations within the foregoing examples which havebeen simplified for purposes of a general explanation of the differenttypes. However, it is to be noted in general that systems which are ofthe “charge depleting” type typically require a battery which has ahigher charge capacity (and thus a higher specific energy) than thosewhich are of the “charge sustaining” type if a commercially acceptabledriving range (miles between recharge) is to be attained in operation.Further and more specific discussion of the various types of HEVsystems, including “series”, “parallel” and “dual mode” types, and ofthe present invention embodied in such systems will be presented below.

In the present application, the phrase “combustion engine” is used torefer to engines running off of any known fuel, be it hydrogen orhydrocarbon based such as gasoline, alcohol, or natural gas, in anycombination.

The use of hybrid drive systems offers critical advantages for both fueleconomy and ultra-low emissions. Combustion engines achieve maximumefficiency and minimal emissions when operated at or near the designpoint speed and load conditions. Small electric motors are capable ofproviding very high peak torque and power. Thus, the ability to use asmall combustion engine operating at maximum efficiency coupled with anelectric motor operating at maximum efficiency offers an outstandingcombination for minimizing emissions, providing excellent fuel economy,and maximizing acceleration.

A key enabling requirement for REV systems is an energy storage systemcapable of providing very high peak power combined with high energydensity while at the same time accepting high regenerative brakingcurrents at very high efficiency. In addition, the duty cycle of a peakpower application requires exceptional cycle life at low depths ofdischarge, particularly in charge depleting systems.

It is important to understand the different requirements for this energystorage system compared to those for a pure electric vehicle. Range isthe critical factor for a practical EV, making energy density thecritical evaluation parameter. Power and cycle life are certainlyimportant, but they are secondary to energy density for an EV. Alightweight, compact, high-capacity battery is the target for pure EVapplications.

In contrast, in REV applications, gravimetric and volumetric powerdensity is the overwhelming consideration. Excellent cycle life from 30to 60% DOD is also more critical than cycle life at 80% DOD as requiredin EV applications. Similarly, rapid recharge is also essential to allowefficient regenerative braking, and charge/discharge efficiency iscritical to maintain battery state of charge in the absence of externalcharging. In addition, thermal management and excellent gasrecombination are important secondary considerations to rapid rechargingand multiple cycling.

Heat generated during charging and discharging NiMH batteries isnormally not a problem in small consumer batteries or even in largerbatteries when they are used singly for a limited period of time. On theother hand, batteries used in HEVs will be subjected to many rapidcharge and discharge cycles during normal operation. Such rapid chargingand discharging will result in significant thermal swings that canaffect the battery performance. The prior art suggests a variety ofsolutions to this problem, such as the following:

U.S. Pat. No. 4,115,630 to Van Ommering, et al., describes a metaloxide-hydrogen battery having bipolar electrodes arranged in a centrallydrilled stack. This patent describes conducting heat generated in theelectrode stack via the hydrogen gas of the cell. In particular, theapplication notes that because heat conduction perpendicular toelectrode plates is 10-20 times smaller than conduction parallel toelectrode plates, cells using flat electrodes must be modifiedsignificantly which makes them unacceptably heavy.

J. Lee, et al. describe resistive heating and entropy heating inlead-acid and nickel/iron battery modules in 133(7) JESOAN 1286 (July,1986). This article states that the temperature of these batteries isdue to resistive heating and entropy changes of the electrochemicalreactions often varies considerably during their operation. They notethat the thermal resistance caused by the cell case plays an importantrole as the cell temperature becomes higher. This reference suggeststhat some additional cooling structure must be added to the battery.

U.S. Pat. No. 4,865,928 to Richter describes a method of removing heatfrom the interior of a high-performance lead acid battery by attaching aU-shaped tube to the negative electrode grid and circulating a coolantthrough the tube.

U.S. Pat. No. 5,035,964 to Levinson, et al., describe attaching a finnedheat sink to a battery and positioned the combination in a chimneystructure. The finned heat sink produces a convective flow of air in thechimney to cool the battery and extend its life.

All of the above cited references suggest methods of removing heat thatrequires the addition of auxiliary apparatus to the battery pack. Nonesuggest how this can be accomplished without modifications that, as U.S.Pat. No. 4,115,630 specifically states, result in an unacceptableaddition to the total weight of the cell.

In all sealed cells, the discharge capacity of a nickel based positiveelectrode is limited by the amount of electrolyte, the amount of activematerial, and charging efficiency. The charge capacity of a NiMHnegative electrode is limited by the amount of active material used,since its charge efficiency is very high, nearly a full state of chargeis reached. To maintain the optimum capacity for a metal hydrideelectrode, precautions must be taken to avoid oxygen recombination orhydrogen evolution before full charge is reached. This becomes acritical problem for batteries in any HEV system that undergo repetitivecharge and discharge cycles. The problem of venting is not new and manymanufacturers have attemped to solve it. Typically the solution hasinvolved the use of a gas consumption electrode (GCE). Typically GCEsare carbon, copper, silver, or platinum prepared in a porous form toprovide a large surface area for gas recombination is the site ofcatalytic oxygen reduction.

U.S. Pat. No. 5,122,426 describes a GCE that has three distinct layers,a hydrophobic electrically non-conductive first layer, a hydrophilicsecond layer, and a hydrophobic third layer. This third layer iselectrically connected to the negative electrode.

Similarly, U.S. Pat. No. 5,128,219 describes a gas consumption electrodecomprising a metallic component, such as Pd, Ni, or Cu, and a film ofactivated carbon, carbon black, and a binder. Use of the described GCEis particularly discussed in a button cell.

While many GCEs are very efficient, their presence decreases the areaavailable for active electrodes and hence decreases the overallvolumetric energy density of the cell. In cells of an HEV system likeall sealed NiMH cells, it is desirable to keep pressures withinacceptable limitations without the necessity of using a GCE.

The foregoing are just a few examples of the differences in batteryrequirements for EV applications and HEV applications. There are alsomany other differences depending upon the particular type of HEV systememployed. These will be discussed later in connection with particularHEV systems. Given the fundamental differences in requirements betweenthe EV and those for an HEV application, it could be expected that thosebatteries currently optimized for use in EV applications will generallynot be suitable for HEV without increasing power density. While thedemonstrated performance of Ovonic EV batteries has been impressive,these cell and battery designs have been optimized for use in pure EVsand therefore do not meet the specific requirements for HEVs.

SUMMARY OF THE INVENTION

An object of the present invention is a power system for a hybridvehicle comprising NiMH batteries having high peak power combined withhigh energy density and excellent cycle life at low depths of discharge.In particular, the present invention provides high peak power incombination with high energy density, a combination which the prior arthas been unable to provide, as will be explained.

Another object of the present invention is a power system for a hybridvehicle comprising Ovonic NiMH batteries having high power combined withhigh energy density, excellent low depth of discharge cycle life, goodthermal management, and excellent gas recombination.

These and other aspects of the present invention are satisfied by ahybrid electric vehicle drive system comprising a combustion engine, anelectric motor and at least one nickel metal hydride battery module forpowering the electric motor, the at least one nickel metal batterymodule having a peak power density in relation to energy density asdefined by the following expression:

P>1,375−15E

where P is the peak power density as measured in Watts/kilogram and F isthe energy density as measured in Watt-hours/kilogram.

Other aspects of the present invention are satisfied by a hybridelectric vehicle incorporating an integrated propulsion system,comprising: a power system comprising a combustion engine and anelectric motor, nickel metal hydride batteries configured for maximumpower and coupled to the power system, and power controlling meansgoverning the series and/or parallel operation of the combustion engineand the electric motor at maximum efficiency for powering the hybridelectric vehicle and providing for the charge and discharge of thenickel metal hydride batteries. Additionally, a regenerative brakingsystem may be coupled to the power controlling means and to provideadditional charging current for the nickel metal hydride batteries.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical depiction of the relationship between peak poweroutput and energy density of typical prior art nickel metal hydridebatteries in comparison to the performance of the Ovonic nickel metalhydride batteries of the present invention for various HEV application;

FIG. 2 is a schematic representation of a series HEV system in which thepresent invention can be embodied; specifically illustrated is thematrix placement of the battery modules into the pack case, the mannerin which the module spacers form coolant flow channels, fluid inlet andoutlet ports, and fluid transport means;

FIG. 3 is a schematic representation of parallel HEV system in which thepresent invention can be embodied;

FIG. 4 is a planar illustration of an electrode for a prismatic Ni—MHbattery with an attached electrode tab; and

FIG. 5 is a stylized depiction of a top view of one embodiment of thefluid-cooled nickel metal hydride battery pack adopted for HEV uses ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

Nickel-metal hydride batteries of the present invention are adapted toprovide increased specific power and recharge rates that areparticularly advantages in HEV applications. These characteristics areprovided while maintaining a high energy density. This is accomplishedin the present invention through, inter alia, the use of positive andnegative electrodes having increased internal conductance. Suchelectrodes are formed by pressing powdered metal-hydride activematerials into highly conductive porous metal substrates. These porousmetal substrate are formed from copper, a copper alloy, or nickel coatedwith copper or a copper alloy. Additionally, the substrate may be platedwith a material that is electrically conductive and will preventcorrosion of the metal substrate in the battery environment, such asnickel.

With reference to FIG. 1, prior art NiMH batteries designed for use inHEV applications have shown a maximum attainable energy density of about75 Wh/Kg while providing a peak power density capable of about 250 W/Kg.This combination of energy density and peak power density is shown atpoint A in FIG. 1. Allowing for some minor engineering improvements, thepeak power density attainable with prior art NiMH batteries at thisenergy density might be increased to about 300 W/Kg with an energydensity of about 70 Wh/Kg, which is shown at point B in FIG. 1. In orderto increase the peak power density of such prior art batteries for usein HEV systems, it is necessary to sacrifice energy density as a tradeoff in order to attain a higher peak power density. This, in turn,decreases the energy density of the battery so that, for example, as thepeak power density is increased from 250 W/Kg to about 650 W/Kg forbetter HEV performance, the energy density is decreased from about 70Wh/Kg to about 45 Wh/Kg, as shown at point C in FIG. 1. Again, allowingfor some engineering improvement, the peak power density attainable at45 Wh/Kg might be increased to about 700 W/Kg, which is point D in FIG.1.

These points A, B, C and C define a band E which represents the upperlimit of the region (which region defines combinations of both highpower density and high energy density) attainable with prior art NiMHbatteries for use in HEV systems. The present invention providesimproved performance in the region yielding a unique combination of bothhigher power density and higher energy density than has been possible toattain in battery modules adapted for use in REV applications.

Taking the upper limits of the shaded band E of FIG. 1, the upper limitof peak power P density attainable for a selected given density E ofprior art NiMH battery modules for use in REV applications can thereforebe represented by the following equation:

P=1,375−15E  Equation (1)

where P is the maximum available peak power density (measured in W/Kg)attainable for a given energy density E (measured in Wh/Kg). The presentinvention permits operation of REV systems of all types at peak powerdensity levels in relation to energy density in the region that liesbeyond these limits of the existing prior art, that is at levels higherthan those defined by the above equation (1).

For example, a battery module embodying the present invention and havingan energy density of about 70 Wh/Kg typically exhibits a peak powerdensity of at least 600 W/Kg (shown at point F in FIG. 1) and can have apeak power density as high as 1,000 W/Kg (shown at point G in FIG. 1).These points establish a band of peak power to energy densityrelationships particularly suited to REV applications and which aresubstantially beyond the capability of prior art NiMH batteries.

To give specific examples, an Ovonic 60 Ah REV battery embodying thepresent invention and having an energy density of about 70 Wh/Kgprovides a peak power of about 600 W/Kg. This is the example shown atpoint F in FIG. 1. In another example, an Ovonic 30 Ah HEV batteryhaving an energy density of about 55 Wh/Kg provides a peak power ofabout 550 W/Kg. This example is shown at point H in FIG. 1. In a thirdexample, an Ovonic 20 Ah REV battery having an energy density of about50 Wh/Kg provides a peak power of about 600 W/Kg. This example is shownat point I in FIG. 1.

Representative HEV systems in which the present invention is applicableare shown in schematic form in FIGS. 2 and 3. FIG. 2 shows a series HEVsystem in which a combustion engine 70 is connected to drive a generator71. The generator 71 is in turn connected to charge a battery 72 whichsupplies electrical power to a drive motor 73. This drive motor 73 isconnected to the vehicle drive system 74 which supplies drive power tothe vehicle wheels.

The battery may be initially charged from a separate power source suchas through an outlet connected to a public utility system. The battery72 is also recharged to some extent by regenerative braking duringdeceleration.

In the parallel type system as shown in FIG. 3, a battery 75 isconnected to supply electrical power to an electric drive motor 76 whichis connectible through a clutch 77 to vehicle drive system 78. Connectedin parallel with the electric drive path formed by the battery 75, themotor 76 and the clutch 77 is a combustion engine 79 which is alsoconnectible to the vehicle drive system 78. The vehicle can driven byeither the electric motor 76 when the clutch 77 alone is engaged or bythe engine 79 when the clutch 80 alone is engaged, or by both the motor76 and the engine 79 simultaneously when both clutches 77 and 80 areengaged at the same time.

In the parallel system as shown in FIG. 3, the combustion engine 79 maybe sized much smaller than would otherwise be required to provideacceptable vehicle acceleration characteristics because the electricmotor 76 can be engaged along with the engine 79 to provide the desiredacceleration. This means that, if the combustion engine is used for theprimary drive mode, it can be operated at a much improved efficiencyunder steady state load and speed conditions.

Various combinations of the electric motor 76 and the combustion engine79 are employed in parallel type systems. For example, in one systemintended for use in city environments, vehicle propulsion is provided bythe electric motor 76 alone when the vehicle is operated within thecity. Outside of the city, the combustion engine 79 may be used forpropulsion purposes. Various other combinations are also employed usingthe parallel type of connection as shown in FIG. 3.

Parallel type systems such as shown in FIG. 3 are also operated ineither the charge sustaining or charge depleting mode as explainedabove. As shown in the diagram of FIG. 3, generative power feedbackduring regenerative braking. Other connections (not shown) can also beprovided to permit the combustion engine 79 to provide recharging powerto the battery 75 to implement a charge sustaining mode of operation.

For example, in the so-called “series-parallel” or “compound” HEVsystem, sometimes referred to as a “dual mode” system, a power splitteris used to take off some of the power from the combustion engine todrive a generator which provides recharging power to the battery.

FIG. 1 has been divided into sectors depicting those regions in whichthe various forms of HEV systems would be operated. In the region CD,for example, systems which are of the charge depleting type wouldtypically be operated. This is because the battery is not rechargedduring operation and the emphasis will thus be on a high energy densityfor maximum range. This region is also referred to as the “rangeextender” region.

For the case of charge sustaining systems, where the battery isrecharged during operation, a lower energy density is accepted and theemphasis is on a higher peak power for improved performance with a lowerenergy density being accepted as a trade off for increase in powerdensity. This region is designated CS is the diagram of FIG. 1. Thisregion is also referred to as the “power assist” region.

Compound or dual mode systems would be operated in the region DS inbetween the regions CD and CS as shown in FIG. 1.

The parameter of peak power is determined in accordance with standardsestablished by the United States Advanced Battery Consortium (USABC).According to these standards, peak power is measured with the batterymodule discharged to 50% depth of discharge. At this condition a currentand corresponding power output which reduces the voltage of the batteryto ⅔ of its open circuit voltage held for a period of ten seconds is thepeak power rating of the battery. This determination is made undernormal temperature conditions in the range of about 30 to 35° C.

The energy density or specific energy E is measured for the batterymodule as designed for use in HEV applications. This determination isalso made under normal temperature conditions in the range of about 30to 35° C.

A battery module is an integral assembly of cells connected together andencased in a casing and having external electrical connections forconnection to an external circuit or load.

As noted above, the present invention enables operation in the higherperformance region above the band E for all HEV system types, i.e.,charge depleting, charge sustaining and dual operation. Prior art NiMHbattery systems for HEV applications are unable to provide performancein this enhance performance region.

The power controlling means of the present invention that governsoperation of the combustion engine and the electric motor at maximumefficiency of said nickel metal hydride batteries can be any knowncontrol device. Preferably, the power controlling means is a solid stateintegrated microelectronic device including AI algorithms thatincorporate appropriate sensors and self-regulating and self-adjustingsub-routines. These permit constant adjustment of control parameters toattain maximum efficiency based on numerous external factors such astype of driving, average driving speed, ambient temperature, etc., aswell as system factors such as engine temperatures, charge/dischargetimes and rates, battery temperatures, fuel consumption, etc.

The electrodes can also include current collection lines on thesubstrate. Such current collection lines have a higher electricalconductivity than the remainder of the substrate. This configurationassures high conductivity pathways from points remote from the currentcollecting tab on the electrode to the current collection tab. Oneembodiment of the current collection line comprises densifying portionsof the porous metal substrate. Another embodiment comprises wires,ribbons or sintered powder electrically attached or embedded into theporous metal substrate. These attached or embedded components can beformed from nickel, copper, a copper alloy, nickel coated with copper ora copper alloy, or a copper material coated nickel.

A primary consideration of the present invention involves improving thepower output of an Ovonic nickel-metal hydride (NiMH) rechargeablebattery. (While reference is made specifically to Ovonic NiMH batteries,the principles described herein are applicable to all types of metalhydride battery systems regardless of their designation.) Generally,power output may be increased by lowering the internal resistance of thebattery. Lowering the internal resistance decreases the power wasted dueto heat dissipation within the battery, thereby increasing the powerwhich is available to drive external loads. The internal resistance of anickel-metal hydride battery can be decreased by increasing theconductivity of the battery components as well as the connectionsbetween the components. More specifically, the internal resistance canbe decreased by increasing the conductivity of both the positive andnegative electrodes of the battery.

The volumetric peak power density of the batteries of the presentinvention is generally ≧1500 W/L, preferably ≧1800 W/L, and mostpreferably ≧2700 W/L. The specific peak power density of batteries ofthe present invention is generally ≧600 W/kg, preferably ≧700 W/kg, andmost preferably ≧1000 W/kg. In batteries of the present invention, it isusually necessary to sacrifice energy density in favor of power density.With this in mind, the volumetric peak energy density of the batteriesof the present invention is generally between 130-250 Wh/L, preferably≧150 Wh/L, and most preferably ≧160 Wh/L.

In general, NiMH batteries employ a negative electrode having an activematerial that is capable of reversible electrochemical storage ofhydrogen. Upon application of an electrical potential across a NiMHbattery, the active negative electrode material is charged by theelectrochemical absorption of hydrogen and the electrochemicalgeneration of hydroxyl ions. The electrochemical reaction at thenegative electrode is as follows:

The negative electrode reactions are reversible. Upon discharge, thestored hydrogen is released to form a water molecule and release anelectron.

The negative electrodes of a nickel-metal hydride battery are generallyformed by pressing powdered active material into a porous metalsubstrate. As discussed, the powdered active material of the negativeelectrode is a hydrogen storage material. The hydrogen storage materialmay be chosen from the Ti—V—Zr—Ni active materials such as thosedisclosed in U.S. Pat. Nos. 4,551,400 (“the '400 Patent”), thedisclosure of which is incorporated by reference. As discussed above,the materials used in the '400 Patent utilize a generic Ti—V—Nicomposition, where at least Ti, V, and Ni are present with at least oneor more of Cr, Zr, and Al. The materials of the '400 Patent aremultiphase materials, which may contain, but are not limited to, one ormore phases with C₁₄ and C₁₅ type crystal structures.

There are other Ti—V—Zr—Ni alloys which may also be used for thehydrogen storage material of the negative electrode. One family ofmaterials are those described in U.S. Pat. No. 4,728,586 (“the '586Patent”), the disclosure of which is incorporated by reference. The '586Patent discloses a specific sub-class of these Ti—V—Ni—Zr alloyscomprising T, V, Zr, Ni, and a fifth component, Cr. The '586 Patentmentions the possibility of additives and modifiers beyond the T, V, Zr,Ni, and Cr components of the alloys, and generally discusses specificadditives and modifiers, the amounts and interactions of the modifiers,and the particular benefits that could be expected from them.

In addition to the materials described above, hydrogen storage materialsfor the negative electrode of a NiMH battery may also be chosen from thedisordered metal hydride alloy materials that are described in detail inU.S. Pat. No. 5,277,999 (“the '999 Patent”), to Ovshinsky and Fetcenko,the disclosure of which is incorporated by reference.

As stated above, the active hydrogen storage material is compressed ontoa porous metal substrate. Generally, the porous metal substrateincludes, but is not limited to, mesh, grid, matte, foil, foam andplate. Preferably, the porous metal substrate used for the negativeelectrode is a mesh or grid. The present invention includes nickel metalhydride batteries having negative and positive electrodes that comprisea porous metal substrate formed from one or more materials selected fromthe group consisting of copper, copper alloy, nickel coated with copper,nickel coated with copper alloy, and mixtures thereof. Preferably, theporous metal substrate is formed from copper or copper alloy.

Alkaline batteries represent an extremely harsh operating environment.In order to protect the electrodes from the harsh environment within thebattery, the porous metal substrate formed from the materials describeabove may be plated with a material that is electrically conductive yetresistant to corrosion in the battery environment. Examples of materialsthat can be used to plate the negative electrode include, but are notlimited to, nickel and nickel alloy.

Using copper or copper alloy to form the porous metal substrate of thenegative electrode has several important advantages. Copper is anexcellent electrical conductor. Hence, its use as a substrate materialdecreases the resistance of the negative electrode. This decreases theamount of battery power wasted due to internal dissipation, and therebyprovides a NiMH battery having increased output power.

Copper is also a malleable metal. Malleability is very important becauseof the expansion and contraction of the negative electrodes duringcharge and discharge cycling of a NiMH battery. The increased pliabilityof the substrate helps prevent electrode breakage as a result of theexpansion and contraction, thereby resulting in improved batteryreliability. Further, copper has excellent thermal conductivity. Ofitself, this fact aids in the temperature management of the batteries ofthe invention. And copper's thermal conductivity tends to furtherenhance the thermal-conductive aspects of the invention described below.

Increased substrate malleability also means that the substrate can morereliably hold the active hydrogen storage material that is compressedonto the substrate surface, thereby improving battery reliability. Thisalso lessens the need to sinter the negative electrodes after thestorage material is compressed onto the substrate surface, therebyreducing the cost and increasing the speed in which the electrodes aremade.

Another way to increase the power output from a nickel-metal hydridebattery is to increase the conductivity of the battery's positiveelectrodes. As in the case of the negative electrodes, this can be doneby appropriately altering the materials from which certain electrodecomponents are made.

Generally, the positive electrode of the nickel-metal hydride battery isformed by pressing a powdered active positive electrode material into aporous metal substrate. NiMH batteries generally employ a positiveelectrode having nickel hydroxide as the active material. The reactionsthat take place at the positive electrode are as follows:

The nickel hydroxide positive electrode is described in U.S. Pat. No.5,344,728 and 5,348,822 (which describe stabilized disordered positiveelectrode materials) and U.S. Pat. No. 5,569,563 and U.S. Pat. No.5,567,549 the disclosures of which are incorporated by reference.

The porous metal substrate of the positive electrode includes, but isnot limited to, mesh, grid, matte, foil, foam and plate. Disclosedherein, is a positive electrode comprising a porous metal substrate thatis formed from one or more materials selected from the group consistingof copper, copper alloy, nickel coated with copper, nickel coated with acopper alloy, and mixtures thereof. Forming the substrate from one ormore of these materials increases the conductivity of the positiveelectrodes of the battery. This decreases the amount of power wasted dueto internal power dissipation, and thereby increases the power output ofthe NiMH battery.

To protect the positive electrode from the harsh battery environment,the porous metal substrate may be plated with a material which iselectrically conductive yet resistant to corrosion in the batteryenvironment. Preferably, the porous metal substrate may be plated withnickel for protection.

The conductivity of the positive electrode may be increased in otherways. The conductivity of the positive electrode can be increased byintroducing lines of higher electrical conductivity into the porousmetal substrate. These “current collection lines” are formed so as tohave a higher electrical conductivity than the remainder of thesubstrate and thus provide high conductivity pathways from points remotefrom the current collection tabs of the positive electrodes.

An embodiment of a positive electrode comprising current collectionlines is shown in FIG. 4. As shown in FIG. 4, attached to the positiveelectrode 1′ is a current collecting tab 2. Generally, the currentcollecting tab 2 is attached to at least one point of the positiveelectrode attached along the top of the positive electrodes so that theymay be easily connected to the positive battery terminal of thenickel-metal hydride battery. The current collecting tab 2 may be formedof any electrically conducting material which is resistant to corrosionfrom the battery environment. Preferably, the current collecting tab 2may be formed of nickel, nickel-plated copper, or nickel-plated copperalloy. Forming the current collecting tab 2 from either nickel-platedcopper or nickel-plated copper alloy rather than from nickel decreasesthe resistance of the tab and increases the power output from thebattery. Tabs formed from either nickel-plated copper or nickel-platedcopper alloy may be connected to the battery terminal via laser welding.

As described, the current collecting lines provide high conductivitypathways from points remote from the current collection tabs. Thecurrent collection lines may be configured in many different ways.Preferably, the current collection lines are configured to minimize theresistance of the positive electrodes and allow the current flowing inthe electrode to reach the collecting tabs with minimal loss of power.Because the current collection lines provide high conductivity pathwaysfor the current, the overall conductivity of the positive electrodes isincreased, thereby reducing the waste of internal power dissipation andincreasing the power output of the battery. One embodiment of aconfiguration of the current collection lines is shown in FIG. 4, wherethe current collecting lines 3 traverse the positive electrode.

The current collection lines are formed in a porous metal substratewhich, as discussed above, includes, but is not limited to, mesh, grid,matte, foil, foam and plate. Preferably, the porous metal substrate isformed from foam. More preferably, the porous metal substrate is formedfrom nickel foam or nickel plated copper foam.

The current collection lines may be implemented in many different ways.In one embodiment, the current collection lines comprise densifiedportions of the porous metal substrate. The densified portions of thematerial are more conductive than the remainder of the material. Whenthe substrate is comprised of foam, the current collection lines can beformed (i.e. the material densified) by compressing the appropriateportions of the foam.

In yet another embodiment, the current collection lines may be formed byconductive powder which is sintered to the porous substrate in theappropriate configuration. The powder may be formed from one or morematerials selected from the group consisting of copper, copper alloy,nickel-plated copper, nickel-plated copper alloy, nickel, nickel coatedwith copper, and nickel coated with copper alloy, and mixtures thereof.

Alternately, in another embodiment, the current collection lines may beformed by first forming densified portions or channels in the poroussubstrate, and then integrating the conductive wire, ribbon or powderinto these densified portions or channels.

In other embodiments, the current collection lines are formed byconductive wires and ribbons that are electrically connected to thesubstrate and appropriately placed to minimize the resistance of theelectrodes. The wires or ribbons may be formed from one or morematerials selected from the group consisting of copper, copper alloy,nickel-plated copper, nickel-plated copper alloy, nickel, nickel coatedwith copper, and nickel coated with copper alloy, and mixtures thereof.

In another embodiment of the present invention, the conductivity of thepositive electrodes is further enhanced by the addition of conductiveadditives added to the nickel hydroxide active electrode material. Suchcurrent conducting additives are chosen from the group consisting ofnickel particles, nickel fibers, graphite particles, nickel platedgraphite particles, nickel plated copper particles, nickel plated copperfibers, nickel flakes, and nickel plated copper flakes.

Another aspect of this invention is a nickel-metal hydride batteryhaving at least one positive electrode of the type disclosed herein. Andyet another aspect of this invention is a nickel-metal hydride batteryhaving at least one negative electrode of the type disclosed herein.

In all batteries, heating occurs during charging and discharging.Because internal resistance in NiMH batteries is low, less heat isgenerated than in many prior art types of batteries. Recent experimentaldata indicates that during overcharge the heat generated by therecombination of oxygen, while not significant in small consumerbatteries, could become a problem with the batteries of REV systemdescribed herein.

Heat would become a particular problem in sealed NiMH batteries havingpasted electrodes and a plastic case in an HEV system application.Recent analysis using a pasted electrode and a plastic case has shownthat the heat generated during overcharge is essentially trapped in thecell where temperatures can reach 80° C. In NiMH batteries, excessiveheat decreases performance and decreases cell life due to separator andseal degradation as well as accelerated degradation of the nickelhydroxide and metal hydride active materials.

Many NiMH batteries currently on the market use pasted metal hydrideelectrodes in order to achieve sufficient gas recombination rates and toprotect the base alloy from oxidation and corrosion. The pastedelectrode typically mixes the active material powder with plasticbinders, such as Teflon, and other nonconductive hydrophobic materialsto the electrode. An unintended consequence of this process is asignificant reduction in the thermal conductivity of the electrodestructure as compared to a structure of the present invention whichconsists essentially of a 100% conductive active material pressed onto aconductive substrate.

In an embodiment of the sealed NiMH batteries that are a component ofthe HEV system of the present invention, the buildup of heat generatedduring overcharge is avoided by using a cell bundle of thermallyconductive NiMH electrode material. This thermally conductive NiMHelectrode material that contains NiMH particles in intimate contact witheach other. Oxygen gas generated during overcharge recombines to formwater and heat at the surface of these particles. In the presentinvention, this heat follows the negative electrode material to thecurrent collector and then to the surface of the case. The thermalefficiency of the bundle of thermally conductive NiMH electrode materialcan be further improved if this electrode bundle is in thermal contactwith a battery case that is also of high thermal conductivity.

In such thermally efficient batteries, the NiMH negative electrodematerial is preferably a sintered electrode such as described in U.S.Pat. Nos. 4,765,598; 4,820,481; and 4,915,898 (the contents of which areincorporated by reference) sintered so that the NiMH particles are inintimate contact with each other.

Yet another aspect of the present invention is a fluid-cooled batterypack systems (as used herein the terms “battery pack” or “pack” refer totwo or more electrically interconnected battery modules). Again, itshould be noted that during cycling of the batteries they generate largeamounts of waste heat. This is particularly true during charging of thebatteries, which in a hybrid vehicle is nearly constant. This excessheat can be deleterious and even catastrophic to the battery system.Some of the negative characteristics which are encountered when thebattery pack systems have no or improper thermal management include: 1)substantially lower capacity and power; 2) substantially increased selfdischarge; 3) imbalanced temperatures between batteries and modulesleading to battery abuse; and 4) lowered cycle life of the batteries.Therefore, it is clear that to be optimally useful the battery packsystems need proper thermal management.

Some of the factors to be considered in the thermal management ofbattery pack systems are 1) all batteries and modules must be keptcooler than 65° C. to avoid permanent damage to the batteries; 2) allbatteries and modules must be kept cooler than 55° C. to get at least80% of the battery's rated performance; 3) all batteries and modulesmust be kept cooler than 45° C. to achieve maximum cycle life; and 4)the temperature difference between individual batteries and batterymodules must be kept below 8° C. for optimal performance. It should benoted that the improvements in the instant invention regulate thetemperature difference between batteries to less than about 2° C.

The thermal management of the battery pack system must provide adequatecooling to insure optimal performance and durability of the Ni—MHbatteries in a wide variety of operating conditions. Ambienttemperatures in the U.S. lie in a wide range from at least −30° C. to43° C. in the lower 49 states. It is necessary to achieve operationalusefulness of the battery packs under this ambient temperature rangewhile maintaining the batteries in their optimal performance range ofabout −1° C. to 38° C.

Nickel-metal hydride batteries show charge efficiency performancedegradation at extreme high temperatures over 43° C. due to problemsresulting from oxygen evolution at the nickel positive electrode. Toavoid these inefficiencies the battery temperature during charge shouldideally be held below 43° C. Nickel-metal hydride batteries also showpower performance degradation at temperatures below about −1° C. due todegraded performance in the negative electrode. To avoid low power, thebattery temperature should be held above about −1° C. during discharge.

As alluded to above, in addition to degraded performance at high and lowtemperatures, detrimental effects can occur as a result of temperaturedifferentials between batteries within a module during charge. Largetemperature differentials cause imbalances in charge efficiencies of thebatteries, which, in turn, can produce state-of-charge imbalancesresulting in lowered capacity performance and potentially leading tosignificant overcharge and overdischarge abuse. To avoid these problemsthe temperature differential between the batteries should be controlledto less than 8° C. and preferably less than 5° C.

Other factors in the design of a fluid-cooled battery pack systeminclude mechanical considerations. For instance, battery and modulepacking densities must be as high as possible to conserve space in theend product. Additionally, anything added to the battery pack system toprovide for thermal management ultimately reduces the overall energydensity of the battery system since it does not contribute directly tothe electrochemical capacity of the batteries themselves. In order tomeet these and other requirements the instant inventors have designedthe fluid-cooled battery pack system of the instant invention.

In its most basic form (an embodiment shown in FIG. 5) the instantfluid-cooled battery pack system 39 includes: 1) a battery-pack case 40having at least one coolant inlet 41 and at least one coolant outlet 42;2) at least one battery module 32 disposed and positioned within thecase 40 such that the battery module 32 is spaced from the case wallsand from any other battery modules 32 within the case 40 to form coolantflow channels 43 along at least one surface of the bundled batteries,the width of the coolant flow channels 43 is optimally sized to allowfor maximum heat transfer, through convective, conductive and radiativeheat transfer mechanisms, from the batteries to the coolant; and 3) atleast one coolant transport means 44 which causes the coolant to enterthe coolant inlet means 41 of the case 40, to flow through the coolantflow channels 43 and to exit through the coolant outlet means 42 of thecase 40. Preferably, and more realistically, the battery pack system 39includes a plurality of battery modules 32, typically from 2 to 100modules, arranged in a 2 or 3 dimensional matrix configuration withinthe case. The matrix configuration allows for high packing density whilestill allowing coolant to flow across at least one surface of each ofthe battery modules 32.

The battery-pack case 40 is preferably formed from an electricallyinsulating material. More preferably the case 40 is formed from a lightweight, durable, electrically insulating polymer material. The materialshould be electrically insulating so that the batteries and modules donot short if the case touches them. Also, the material should be lightweight to increase overall pack energy density. Finally, the materialshould be durable and capable of withstanding the rigors of the batterypack's ultimate use. The battery pack case 40 includes one or morecoolant inlets 41 and outlets 42, which may be specialized fluid ports,where required, but are preferably merely holes in the battery pack case40 through which cooling-air enters and exits the battery pack.

The fluid cooled battery-pack system 39 is designed to useelectrically-insulating coolant, which may be either gaseous or liquid.Preferably the coolant is gaseous and more preferably the coolant isair. When air is used as the coolant, the coolant transport means 44 ispreferably a forced-air blower, and more preferably a blower whichprovides an air flow rate of between 1-3 SCFM of air per cell in thepack.

The blowers do not need to continuously force cooling air into thebattery pack, but may be controlled so as to maintain the battery packtemperatures within the optimal levels. Fan control to turn the fan onand off and preferably to control the speed of the fan is needed toprovide for efficient cooling during charging, driving, and idle stands.Typically, cooling is most critical during charge, but is also neededduring aggressive driving. Fan speed is controlled on the basis of thetemperature differential between the battery pack and ambient, as wellas on the basis of absolute temperature, the latter so as not to coolthe battery when already it is already cold or so as to provide extracooling when the battery nears the top of its ideal temperature range.For nickel-metal hydride batteries, fans are also needed in idle periodsafter charge. Intermittent cooling is needed to provide for efficientcooling under this condition and results in net energy savings bykeeping self discharge rates below fan power consumption. A typicalresult shows a fan on time of 2.4 hours after the initial post chargecooldown. Typically the normal fan control procedure (described below)works well in this scenario. Fan control allows for the use of powerfulfans for efficient cooling when needed without the consumption of fullfan power at all times, thus keeping energy efficiency high. The use ofmore powerful fans is beneficial in terms of maintaining optimal packtemperature which aids in optimization of pack performance and life.

One example of a fan control procedure provides that, if the maximumbattery temperature is over 30° C. and the ambient temperature is lower(preferably 5° C. or more lower) than the maximum battery temperaturethen the fans will turn on and circulate cooler air into the coolantchannels.

The flow rate and pressure of the cooling fluid needs to be sufficientto provide sufficient heat capacity and steady state removal of heat atthe maximum anticipated sustained heat generation rate to result in anacceptable temperature rise. In typical Ni-MH battery packs, with 5-10 Wper cell generated during overcharge (maximum heat generation), a flowrate of 1-3 CFM of air per cell is needed to provide adequate coolingsimply on the basis of the heat capacity of air and achieving anacceptable temperature rise. Radial blower type fans may be used toprovide the most effective airflow for thermal management. This is dueto the higher air pressure generated by these fan types as contrastedwith that generated by axial fans. Generally, a pressure drop of atleast 0.5″ of water is required at the operating point of the fan asinstalled in the pack. To produce this pressure drop at high flow ratesgenerally requires a fan static pressure capability of 1.5″ to 3″ ofwater.

In addition to using the fans to cool the battery pack when it is hot,the fans can heat the battery pack when it is too cold. That is, if thebattery pack is below its minimum optimal temperature, and the ambientair is warmer than the battery pack, the fans may be turned on to drawwarmer ambient air into the battery pack. The warmer air then transfersits thermal energy to the battery pack and warms it to at least the lowend of the optimal range of temperature.

One or more coolant transport means 44 can be positioned at the coolantinlet 41 to force fresh coolant into the battery pack case 40, throughcoolant flow channels 43, and out of the coolant outlet 42.Alternatively, one or more coolant transport means 44 can be positionedat the coolant outlet 42 to draw heated coolant out of the battery packcase 40, causing fresh coolant to be drawn into the battery pack case 40via the coolant inlet 41, and to flow through the coolant flow channels43.

The coolant may flow parallel to the longest dimension of the coolantflow channels 43 (i.e. in the direction of the length of the batterymodules) or, alternatively, it may flow perpendicular to the longestdimension of said coolant flow channels 43, (i.e. in the direction ofthe height of the batteries). As it flows through the cooling channels43, the coolant heats up. Therefore, it is preferable that the fluidflow perpendicular to the longest dimension of the cooling channels 43.This is because as the coolant heats up, the temperature differencebetween the batteries and the coolant decreases and therefore, thecooling rate also decreases. Thus the total heat dissipation is lowered.To minimize this effect, the coolant flow path should be the shorter ofthe two, i.e. along the height of the batteries.

While air is the most preferred coolant (since it is readily availableand easy to transport into and out of the case) other gases and evenliquids may be used. Particularly, liquid coolants such as freon orethylene glycol, as well as other commercially available fluorocarbonand non-fluorocarbon based materials may be used. When these other gasesor liquids are used as the coolant, the coolant transport means 44 maypreferably be a pump. When using coolants other than air, the coolanttransport means may preferably include a coolant return line attached tothe coolant outlet 42 which recycles heated coolant to a coolantreservoir (not shown) from which it is transferred to a coolant heatexchanger (not shown) to extract heat therefrom and finally redeliveredto the coolant pump 44 for reuse in the cooling of the battery pack 39.

The optimized coolant flow channel width incorporates many differentfactors. Some of these factors include the number of batteries, theirenergy density and capacity, their charge and discharge rates, thedirection, velocity and volumetric flow rate of the coolant, the heatcapacity of the coolant and others. It has been found that independentof most of these factors, it is important to design the cooling channels43 to impede or retard the cooling fluid flow volume as it passesbetween the modules. Ideally, the retardation in flow is predominantlydue to friction with the cell cooling surfaces, which results in a flowreduction of 5 to 30% in flow volume. When the gaps between modules formthe major flow restriction in the cooling fluid handling system, thisproduces a uniform and roughly equal cooling fluid flow volume in thegaps between all modules, resulting in even cooling, and reducing theinfluence of other flow restrictions (such as inlets or exits) whichcould otherwise produce nonuniform flow between the modules.Furthermore, the same area of each cell is exposed to cooling fluid withsimilar velocity and temperature.

Battery modules are arranged for efficient cooling of battery cells bymaximizing the cooling fluid velocity in order to achieve a high heattransfer coefficient between the cell surface and the cooling fluid.This is achieved by narrowing the intermodule gap to the point that thecooling fluid also helps raise the heat transfer coefficient as theshorter distance for heat transfer in the cooling fluid raises the cellto fluid temperature gradient.

The optimal coolant flow channel width depends on the length of the flowpath in the direction of flow as well as on the area of the coolant flowchannel in the plane perpendicular to the flow of the coolant. There isa weaker dependence of optimal gap on the fan characteristics. For air,the width of the coolant flow channels 43 is between about 0.3-12 mm,preferably between 1-9 mm, and most preferably between 3-8 mm. Forvertical air flow across a module 7 inches high, the optimal achievablemean module spacing (width of the coolant flow channels 43) is about 3-4mm (105 mm centerline spacing). For horizontal air flow lengthwiseacross 4 modules 16 inches long in a row for a total distance of 64inches, the optimal achievable mean module spacing (width of the coolantflow channels 43) is about 7-8 mm (109 mm centerline spacing). Slightlycloser intermodule spacing at the far end of this row will result in ahigher airflow rate and consequently a higher heat transfer coefficient,thus compensating for the higher air temperature downstream. A secondaryinlet or series of inlets partway along the horizontal coolant flow pathcan also be used as a means of introducing additional coolant, thusmaking the heat transfer between the battery cells and the coolant moreuniform along the entire flow path.

In should be noted that the term “centerline spacing” is sometimes usedsynonymously with coolant flow channel width. The reason for this isthat the quoted coolant flow channel widths are average numbers. Thereason for this averaging is that the sides of the battery modules whichform the flow channels 43 are not uniformly flat and even, the bandingwhich binds the modules together and the sides of the batteriesthemselves cause the actual channel width to vary along its length.Therefore, it is sometimes easier to describe the width in terms for thespacing between the centers of the individual modules, i.e. thecenterline width, which changes for batteries of different sizes.Therefore, it is generically more useful to discuss an average channelwidth, which applies to battery modules regardless of the actual batterysize used therein.

To assist in achieving and maintaining the proper spacing of the moduleswithin the pack case and to provide electrical isolation between themodules, each module includes coolant-flow-channel spacers 37 which holdthe modules 32 at the optimal distance from any other modules 32 andfrom the battery pack case 40 to form the coolant flow channels 43. Asdisclosed above, the coolant-flow-channel spacers 37 are preferablypositioned at the top and bottom of the battery modules 32, providingprotection to the corners of the modules 32, the battery terminals 7, 8and the electrical interconnects 25. More importantly, tabs on the sidesof the spacers 38 hold the modules at the optimal distance apart. Thespacers 37 are preferably formed from a light weight, electricallynon-conductive material, such as a durable polymer. Also, it isimportant to the overall pack energy density that the spacers include aslittle total material as possible to perform the required function andstill be as light as possible.

As mentioned above Ni—MH batteries operate best in a specifictemperature range. While the cooling system described above enables thebattery pack systems of the instant invention to maintain operatingtemperatures lower than the high temperature limit of the optimal range(and sometimes to operate above the lower temperature limit of theoptimal range, if the ambient air temperature is both warmer than thebattery and warmer than the lower temperature limit of the optimalrange), there are still times when the battery system will be colderthan the lower limit of optimal temperature range. Therefore, there is aneed to somehow provide variable thermal insulation to some or all or ofthe batteries and modules in the battery pack system.

In addition to the cooling systems described above, another way tothermally control the battery pack systems of the instant invention isby the use of temperature dependant charging regimens. Temperaturedependent charge regimens allow for efficient charging under a varietyof ambient temperature conditions. One method involves charging thebatteries to a continuously updated temperature dependent voltage lidwhich is held until the current drops to a specified value after which aspecified charge input is applied at constant current. Another methodinvolves a series of decreasing constant current or constant power stepsto a temperature compensated voltage limit followed by a specifiedcharge input applied at a constant current or power. Another methodinvolves a series of decreasing constant current or constant power stepsterminated by a maximum measured rate of temperature rise followed by aspecified charge input applied at a constant current or power. Use oftemperature dependant voltage lids ensures even capacity over a widerange of temperatures and ensures that charge completion occurs withminimal temperature rise. For example, use of fixed voltage charge lidsresults in an 8° C. temperature rise. Absolute charge temperature limits(60° C.) are required for this battery to avoid severe overheating whichcan occur in the case of simultaneous failure of charger and coolingsystem. Detection of rate of change of voltage with respect to time(dV/dt) on a pack or module basis allows a negative value of dV/dt toserve as a charge terminator. This can prevent excessive overcharge andimproves battery operating efficiency as well as serving as anadditional safety limit.

As discussed above, in addition to having an upper limit on theoperational temperature range of the instant batteries, there is also alower limit. As also discussed above the cooling system can be used as aheating system. However, it is much more likely that if the battery packtemperature is low, the ambient temperature will also be low, andprobably lower than the battery pack temperature. Therefore, there willbe times during operational use of the battery pack system when it willbe advantageous to thermally insulate the batteries from the ambient.However, the need for thermal insulation will not be constant and mayvary dramatically in only a matter of a very short time period.Therefore, the thermal insulation need will also be variable.

In order to accommodate this variable need for thermal insulation, theinstant inventors have devised a means for providing variable thermalinsulation. The inventive variable thermal insulation means can be usedon individual batteries, battery modules and battery pack systems alike.

In its most basic form, the means provides variable thermal insulationto at least that portion of the rechargeable battery system which ismost directly exposed to said ambient thermal condition, so as tomaintain the temperature of the rechargeable battery system within thedesired operating range thereof under variable ambient conditions.

To provide this variable thermal insulation, the inventors have combinedtemperature sensor means and compressible thermal insulation means. Whenthe temperature sensor indicates that the ambient is cold, the thermalinsulation is positioned in the needed areas to insulate the affectedareas of the battery, module or battery pack system. When the ambient iswarmer, the temperature sensor causes the thermal insulation to bepartly or wholly compressed such that the insulation factor provided tothe battery system by the compressible insulation is partially ortotally eliminated.

The thermal sensors may be electronics which variably increase ordecrease the insulation. The thermal sensors may be electronic sensorswhich feed information to piston devices which variably increase ordecrease the compression upon a compressible foam or fiber insulation.Alternatively, (and more preferably from an electrical energyutilization and mechanical reliability point of view,) the sensor andcompression devices may be combined in a single mechanical device whichcauses variable compression upon the thermal insulation in directreaction to the ambient thermal condition. Such a combinedsensor/compression device and be formed from a bimetallic material suchas the strips used in thermostats. Under low ambient temperatures, thebimetallic device will allow the thermal insulation to expand into placeto protect the battery system from the cold ambient conditions, but whenthe temperature of the battery or ambient rises, the bimetal devicecompresses the insulation to remove its insulating effect from thebattery system.

While the variable thermal insulation can be used to completely surroundthe entire battery, module or battery pack system, it is not alwaysnecessary to do so. The variable thermal insulation can be just aseffective when it only insulates the problems spots of the system. Forexample, in the battery modules and pack systems of the instantinvention, which employ ribbed end plates, it may only be necessary formodules which are most directly influenced by low temperature ambientconditions. These ambient conditions may cause large temperatureimbalances between the batteries of the module(s) and as a resultdegrade the performance of the module or pack system. By providingvariable insulation to the affected end(s) of the module(s) thetemperature differential between the batteries can be reduced oreliminated and the overall temperature of the module(s) can becontrolled.

The battery case of the present invention is preferably constructed of ametallic material such as steel. In a preferred embodiment, the metallicmaterial is stamped, embossed, or shaped to form pressure containingsurfaces that counter the internal pressure of the sealed battery andthus prevent bulging of the case. Bulging is detrimental to individualbatteries because it alters the electrolyte distribution and spatialorientation of the electrodes and separators. Alternatively, acylindrical metallic case can be used.

In all commercial, sealed, metal hydride batteries, the positiveelectrode is designed to be capacity limited. This means that thepositive electrode reaches full charge before the negative electrode.When this occurs, oxygen gas evolves at the positive electrode inproportion to the current supplied. In overcharge, all current isproducing oxygen gas. In order for the battery to remain sealed, theremust be a recombination mechanism for the oxygen gas that is evolved.

One recombination mechanism involves the diffusion of oxygen gasgenerated at the positive electrode through the separator to the surfaceof the metal hydride electrode where it recombines. The rate limitingstep of this mechanism is the diffusion of the oxygen gas through theelectrolyte film to reach the surface of the metal electrode. Once theoxygen gas reaches the surface of the electrode, gas recombination israpid. If, however, the oxygen must diffuse through a thick film ofelectrolyte on the surface of the negative electrode, gas recombinationrates will be slowed significantly. Thus, the rate of the reaction isproportional to the amount of electrolyte at the surface of theelectrode. This amount is referred to as the film thickness of theelectrolyte.

An additional aspect of the present invention is a hydrophobic treatmentthat acts to significantly decrease this film thickness. The describedhydrophobic treatment produces a thin electrolyte film precisely whereit is the most beneficial, at the surface of the metal hydride negativeelectrode.

The present invention recognizes that a hydrophobic treatment is mostimportant at the outer surfaces of the metal hydride electrode and, inparticular, at the metal-electrolyte interface. The present inventioninvolves a small thin coating on the surface of either the negativeelectrode or the surface of the separator in contact with the negativeelectrode. This provides a degree of hydrophobicity where it is needed.The coating of the present invention has a tremendous advantage over theprior art because while the surface of the negative electrode isrendered hydrophobic, the interior remains unaffected. This is becausethe gas state combination occurs only on the outer surface of thenegative. Thus the presence of a hydrophobic interior, as in the priorart, is actually detrimental to electrolyte absorption rates, overallelectrolyte absorption, power, cycle life, low temperature, and otherperformance parameters related to the negative electrode.

It is common for manufacturers of NiMH batteries to mix an organicbinder, such as polytetrafluoroethylene (PTFE), with the metal hydridenegative electrode alloy powder to prevent cracking and loss of themetal hydride materials. Such a formulation results in hydrophobicmaterial in the bulk of the electrode (thereby increasing electroderesistance) and the resulting hydrophobicity reduces the effectivenessof the initial etch in removing surface impurities. In addition,hydrophobic binders in the bulk reduce electrolyte absorption whichlowers cycle life, decreases conductivity, and takes up space.

Contrary to the teachings of the present application, JP A 4-277467teaches making the electrode surface hydrophilic by spraying it withalcohol in order to improve the internal pressure.

Unexpectedly, the inventors of the present invention found that inaddition to using a negative electrode where the surface facing theseparator had been treated to render it hydrophobic it was also possibleto attain similar results by using a separator and an untreated negativeelectrode where the surface of separator facing the negative electrodehad been treated to make it hydrophobic. Without wishing to be bound bytheory, it is believed that hydrophobic material on the surface of theseparator facing the negative electrode is in such intimate contact withthe negative electrode that it reduces the film thickness of theelectrolyte on the electrode as if the negative electrode itself hadbeen treated.

While at first glance it might appear advantageous to treat the surfaceof the negative electrode and the surface of the separator facing thenegative electrode to render them both hydrophobic, the inventors havefound that this is not effective. When both surfaces are treated, thethickness of the resulting hydrophobic material is so great that oxygenrecombination is significantly slowed.

This is the case in JP A 5-242908 which describes using a layer of PTFEbetween the negative electrode and the separator (effectively treatingboth the surface of the negative electrode and the surface of theseparator). While JP A 5-242908 discusses the advantages of oxygenrecombination on the electrode, a table in JP A 5-242908 shows cellpressures reduced only to a range of from 81-114 psi. (The temperatureof the cells is not indicated.) These pressures are much greater thanthe pressures in cells of the present invention, as shown in Table 1,below. The use of a coated electrode or separator as described in thepresent invention, avoids the problems inherent in an extra layer. Acoated electrode according to the present invention simplifies andreduces the cost of assembly because the coating can be applied prior toassembly. Using a thin film layer of PTFE between the separator and theelectrode would generate a variety of problems during assembly. Forexample, stretching could produce non-uniform porosity that wouldproduce non-uniform gas recombination and diffusion rates. A coatedelectrode effectively permits the use of a much thinner hydrophobiclayer so that uniform and rapid oxygen recombination is encouragedwithout impeding diffusion rates.

The present invention is effective with all types of battery systems inwhich oxygen is evolved at the positive electrode during overcharge. Thepresent invention is particularly useful with nickel metal hydridesystems (such as the ones commonly referred to as Ovonic systems, AB₂systems, AB₅ systems, and mischmetal systems) Most particularly, thepresent invention is useful with alloys of the type described incopending U.S. patent application Ser. No. 08/259,793, filed Jun. 14,1994, titled ELECTROCHEMICAL HYDROGEN STORAGE ALLOYS AND BATTERIESFABRICATED FROM MG CONTAINING BASE ALLOYS, now U.S. Pat. No. 5,506,069.

The present invention is particularly useful in batteries of the HEVsystem of the present invention because such batteries must undergonumerous quick charge/discharge cycles. This is because quick chargingresults in earlier oxygen gas generation. In batteries that are beingquick charged it is also important that the oxygen generated duringthese periods of overcharge be recombined quickly to prevent venting andloss of capacity. In addition, the present invention is particularlyeffective at high temperatures, which increases its usefulness inbatteries that are going to be quick charged. Obviously, the use of thethermal management system described above will increase the efficienciesof such gas recombination.

While a 1% PTFE suspension is specifically demonstrated below, anysuitable hydrophobic treatment may be used that will reduce the filmthickness of the electrolyte at the surface of the metal hydridenegative electrode. Cells of the present invention suffer no performancetradeoffs in cycle life, power, charge retention, or low temperatureperformance as a result of the hydrophobic treatment.

While any metal hydride alloy may be used, cells of the presentinvention are preferably fabricated from low pressure negative electrodematerials such as those described in U.S. Pat. No. 5,277,999, thecontents of which are incorporated by reference. Such hydrogen storagealloys have the composition

(Base Alloy)_(a)Co_(b)Mn_(c)Al_(d)Fe_(e)La_(f)Mo_(g)

where Base Alloy represents a disordered multicomponent alloy having atleast one structure selected from the group consisting of amorphous,microcrystalline, polycrystalline (lacking long-range compositionalorder with three or more phases of the polycrystalline structure), andany combination of these structures; b is 0 to 7.5 atomic percent,preferably 4 to 7 atomic percent; c is 0 to 8.5 atomic percent,preferably 6 to 8 atomic percent; d is 0 to 2.5 atomic percent,preferably 0.1 to 2 atomic percent; e is 0 to 6 atomic percent,preferably 1 to 3 atomic percent or 5.3 to 6 atomic percent; f is 0 to4.5 atomic percent, preferably 1 to 4 atomic percent; g is 0 to 6.5atomic percent, preferably 0.1 to 6 atomic percent, most preferablyabout 6 atomic percent; b+c+d+e+f+g>0; and a+b+c+d+e+f+g=100 atomicpercent. A preferred formulation of this Base Alloy contains 0.1 to 60atomic percent Ti, 0.1 to 25 atomic percent Zr, 0.1 to 60 atomic percentV, 0.1 to 57 atomic percent Ni, and 0.1 to 56 atomic percent Cr and b is4 to 7 atomic percent; c is 6 to 8 atomic percent; d is 0.1 to 2 atomicpercent; e is 1 to 2 atomic percent; f is 0.1 to 4 atomic percent; and gis 0.1 to 6 atomic percent; b+c+d+e+f+g>0; and a+b+c+d+e+f+g=100 atomicpercent.

While any positive electrode material compatible with metal hydroxidenegative electrodes may be used (such as nickel hydroxide), the positiveelectrodes of the present invention are preferably of the type describedin U.S. Pat. Nos. 5,344,782, 5,348,822, 5,523,182, 5,569,562, and5,567,549. These electrodes are locally ordered, disordered, highcapacity, long cycle life positive electrodes comprising a solidsolution nickel hydroxide electrode material having a multiphasestructure and at least one compositional modifier to promote themultiphase structure. The multiphase unit cell comprising spacedlydisposed plates with at least one ion incorporated around the plates,the plates having a range of stable intersheet distances correspondingto a 2⁺ oxidation state and a 3.5⁺, or greater, oxidation state. The atleast one compositional modifier is a metal, a metallic oxide, ametallic oxide alloy, a metal hydride, and/or a metal hydride alloy.Preferably the at least one compositional modifier is chosen from thegroup consisting of Al, Bi, Co, Cr, Cu, Fe, Ln, LaH₃, Mn, Ru, Sb, Sn,TiH₂, TiO, Zn.

The separators and bags of the present material are made from materialdescribed in detail in U.S. Pat. No. 5,330,861, the contents of whichare incorporated by reference. Described in detail in this applicationare electrolyte retentive nylon and wettable polypropylene materialsthat are non-reactive with H₂ gas and alkaline electrolyte. Theretentive nylon material is capable of absorbing and retaining moreelectrolyte solution than standard nylon separators. The wettablepolypropylene separators are grafted polypropylene material that retainand absorb electrolyte so that particles, barbs, and residues are notproduced. Grafted polypropylene material is preferably used for both theseparators and the bags of the cells of the present invention.

While the improvements of the battery electrodes described herein aredirected toward both the positive and the negative electrodes, this isin no way intended to be limiting. Thus the formation of batteries ofthe invention comprising sintered negative electrodes combined withenhanced conductivity positive electrodes, or prior art pasted negativeelectrodes combined with enhanced conductivity positive electrodes, orenhanced conductivity negative electrodes combined with prior artpositive electrodes, or enhanced conductivity negative electrodescombined with enhanced conductivity positive electrodes are all intendedto be within the scope of the present invention. (The phrase “enhancedconductivity” as used herein is intended to specifically refer to thenegative or positive electrodes of the batteries of the presentinvention.)

EXAMPLES Example 1

Cells embodying those of the present REV system and those of the priorart are described in Table 1, below.

TABLE 1 Comparison HEV Prototype HEV Optimized Prototype power density(W/L) 1300 1600 2700 specific power (W/kg) 600 600 1000 energy density(Wh/L) 120 190 160 specific energy (Wh/Kg) 55 70 60 negative electrodeconstruction pasted Cu substrate Cu substrate, thin electrodes negativecurrent collector nickel copper copper negative alloy composition mischmetal V₁₈Ti₁₅Zr₁₈Ni₂₉Cr₅Co₇Mn₈ V₁₈Ti₁₅Zr₁₈Ni₂₉Cr₅Co₇Mn₈ positiveelectrode pasted Ni(OH)₂ pasted γ-phase pasted γ-phase Ni(OH)₂ thinNi(OH)₂ with conductive thick additives thin separator polypropylenepolypropylene polypropylene thin thin thin case plastic stainless steelstainless steel aspect ratio square square square top plastic stainlesssteel stainless steel tabs thick thick/laser welded thick/laser welded

As can be seen from Table 1, the embodiments of the invention, the HEVprototype cells and HEV optimized prototype cells represent improvementsover the comparison cells in accordance with the prior art. Inparticular, the REV optimized prototype embodies the most dramaticimprovements.

Table 1 shows that the Cu substrate of the invention provides theimproved current conduction essential for reducing internal resistance.Similarly, the use of conductive additives, such as nickel fibers,nickel plated graphite particles, nickel plated copper particles, nickelplated copper fibers, or the use of a conductive mat embedded in thepasted negative electrode material all contribute to the conductivity ofthe positive electrode. In addition, the use of thick tabs that arelaser welded assures that the improved conductivity of the electrodes isnot lost at the collection points. Alternately, negative electrodeshaving the composition Ti₁₀Zr₂₈Ni₃₆Cr₅Co₅Mn₁₆ may be used.

Example 2

The impact of the thermally conductive electrodes of the presentinvention can be evaluated independently. Comparison cells and thermallyconductive cells were fabricated as described in Table 2.

TABLE 2 Comparison Cell Thermally conductive cells capacity 100 Ah 100Ah energy density ≈70 Wh/kg ≈70 Wh/kg negative electrode pastedsintered, compacted construction negative alloy misch metalV₁₈Ti₁₅Zr₁₈Ni₂₉Cr₅Co₇Mn₈ composition positive electrode Ni(OH)₂ pastedonto Ni(OH)₂ pasted onto foam substrate foam substrate case plasticstainless steel top plastic stainless steel temperature after 80° C. 34°C. cycling(charge/ discharge cycling at C/10 overcharge to 120% ofcapacity

Example 3

Cells (1-7 in Table 3, below) were fabricated as described in U.S. Pat.No. 5,330,861 using a high loft polypropylene separator and negativeelectrode alloy having the following composition:

V₁₈Ti₁₅Zr₁₈Ni₂₉Cr₅Co₇Mn₈

except that the separators were sprayed with 1% aqueous solutions ofPTFE on the side facing the negative electrode prior to assembling thecell. The control cells (designated C1-C7 in Table 3, below) wereassembled using untreated separators.

These cells were charged and discharged at the indicated temperatures.The cells having the 1% PTFE coating on the surface of the separatordemonstrated a consistent pressure reduction. As can be seen, thiseffect is even more pronounced at elevated temperatures and represents asignificant improvement over the prior art. Table 3 also shows thatcells of the present invention suffer no tradeoffs in terms of capacity.

TABLE 3 Temperature Overcharge Capacity (° C.) Pressure (psi) (Ah) C1  032 4.77 C2 10 39 4.58 C3 20 89 4.46 C4 30 113  4.53 C5 40 136  4.53 C650 175  4.45 C7 60 138  3.97 1% Teflon 1  0 46 4.76 2 10 44 4.45 3 20 424.47 4 30 47 4.52 5 40 55 4.54 6 50 75 4.49 7 60 63 4.02

It is obvious to those skilled in the art that additional combinationsof the components described above can be made without departing fromspirit and scope of the present invention. For example, extensiveaddition of conductive components such as metallic nickel or copperpowder to a pasted electrode is anticipated. The discussion anddescription of this specification are merely illustrative of particularembodiments of the invention and are not meant as limitations upon theinvention. It is the following claims, including all equivalents, thatdefine the scope of the invention.

While the invention has been described in connection with preferredembodiments and procedures, it is to be understood that it is notintended to limit the invention to the preferred embodiments andprocedures. On the contrary, it is intended to cover all alternatives,modifications and equivalence which may be included within the spiritand scope of the invention as defined by the claims below.

We claim:
 1. A hybrid electric vehicle drive system, comprising: acombustion engine; an electric motor; and at least one nickel-metalhydride battery module providing electric power to said electric motor,said at least one nickel-metal hydride battery module having an internalresistance effective to provide a peak power density in relation to anenergy density as defined by: P>1,375−15E, with P greater than 600Watts/kilogram, where P is the peak power density as measured inWatts/kilogram and E is the energy density as measured inWatt-hours/kilogram.
 2. The drive system of claim 1, further includingmeans for connecting and disconnecting said combustion engine and saidelectric motor in driving relationship to said electric vehicle.
 3. Thedrive system of claim 2, further including control means for operatingsaid at least one battery module in a charge depleting mode.
 4. Thedrive system of claim 2, further including control means for operatingsaid at least one battery module in a charge sustaining mode.
 5. Thedrive system of claim 1, wherein said energy density is at least 70Watt-hours/kilogram.
 6. The drive system of claim 1, further comprisinga cooling system for cooling said at least one nickel-metal hydridebattery module.
 7. The drive system of claim 1, wherein said peak powerdensity is greater than 700 watts/kilogram.
 8. The drive system of claim1, wherein said peak power density is at least 1000 watts/kilogram. 9.The drive system of claim 1, wherein said at least one battery moduleincludes negative electrodes having porous metal substrates formedsubstantially of copper.
 10. A hybrid electric vehicle incorporating anintegrated propulsion system, comprising: a power system comprising: acombustion engine, and an electric motor; and at least one nickel-metalhydride battery module coupled to said power system and providingelectric power to said electric motor, said battery module having aninternal resistance effective to provide a peak power density inrelation to an energy density as defined by: P>1,375−15E, with P greaterthan 600 Watts/kilogram, where P is the peak power density as measuredin Watts/kilogram and E is the energy density as measured inWatt-hours/kilogram.
 11. The hybrid electric vehicle of claim 10,further comprising: a regenerative braking system providing chargingcurrent for said at least one nickel-metal hydride battery module. 12.The hybrid electric vehicle of claim 10, further comprising a coolingsystem for cooling said at least one nickel-metal hydride batterymodule.
 13. The hybrid electric vehicle of claim 10, wherein said peakpower density is greater than 700 watts/kilogram.
 14. The hybridelectric vehicle of claim 10, wherein said energy density is at least 70Watt-hours/kilogram.
 15. The hybrid electric vehicle of claim 10,wherein said peak power density is at least 1000 Watts/kilogram.